jabber: fix regression with gchecksum in jabber_cache_add (#186)
This crashed with "The checksum '%s' has been closed and cannot be
updated anymore." (which is also a user after free)
Issue introduced in 1bdc6696aefb617873b56001f81d649d6ca3aa8e which is
not in any released version.
There was this md5_digest_keep() function that freed a copy of the
checksum instead of the one you pass. The code that it was replaced with
ignored this distinction, and since jabber_cache_add() needs to reuse the
GChecksum this led to this issue.
No other place in the bitlbee code used this function.
